Hey All,

I have a 91 turboed miata, using most of a Greddy kit and a Flyin Miata piggyback ECU. I keep running into an issue with CEL's coming on on the highway. I however do not feel any change to how the car is running. This problem has not occurred on backroads.

I tried replacing my O2 sensor, since I was pulling codes 15, 17 and 34 (O2 sensor and ISC). I replaced my O2 sensor since the wire got melty from sitting directly next to the manifold. I wrapped the new O2 sensor wire with 2 layers of wire heat shroud. I cleared the codes and went out to do another test, and about 30 minutes into highway driving at ~65mph I got the CEL again. The car continued to run normal, and the WB O2 sensor I use to monitor AFR's said I was cruising at about 14.5. All temps, boost readings and AFR's seemed normal. When I got home I pulled the code and got 15 and 34.

I am at a loss at how to go about trouble shooting this. Could the O2 sensor be malfunctioning from sitting next to the manifold and getting too hot? Also, what would that CEL combined with the ISV valve CEL indicate? It only happens about 30 minutes into highway cruising. Thank you all for the help in advance!